The recent advent of the use of Automatic Dependent Surveillance-Broadcast (ADS-B), an advanced air ground traffic control system, has facilitated the integration of this system with the pre-existing Traffic Alert and Collision Avoidance System (TCAS).
ADS-B is a technology which allows aircraft to broadcast information such as identification, position, altitude. This broadcast information may be directly received and processed by other aircraft or received and processed by ground systems for use in improved situational awareness, conflict avoidance and airspace management. ADS-B incorporates the use of Global Positioning System (GPS) or other similar navigation systems as a source of position data. By using GPS or the like, ADS-B has the capacity to greatly improve the efficiency and safety of the National Airspace System.
ADS-B provides for an automatic and periodic transmission of flight information from an in-flight aircraft to either other in-flight aircraft or ground systems. The ADS-B transmission will typically comprise information items such as altitude, flight ID, GPS (Global Positioning System) position, velocity, altitude rate, etc. The transmission medium for ADS-B can implement VHF, 1090 MHz (Mode S), UHF (UAT), VDLM4 or a combination of systems.
The TCAS system was implement by the Federal Aviation Administration (FAA) in the 1980's to provide aircraft with an on-board collision avoidance system. Since TCAS's inception it has become a standard for air traffic safety in the United States and abroad. There are two different versions of TCAS, for use on different classes of aircraft. The first, TCAS I, indicates the bearing and relative altitude of all aircraft within a selected range (generally 10 to 20 miles). With color-coded symbols, the Cockpit Display of Traffic Information (CDTI) (i.e., the display) indicates which of the aircraft pose a potential threat. This constitutes the traffic advisory (TA) portion of the system. When pilots receive a TA, they must visually identify the intruding aircraft and may alter their plane's altitude by up to 300 feet. TCAS I does not offer solutions for avoidance of a potential threat, but does supply pilots with important data so that they can determine the best course of action.
In addition to a traffic display, the more comprehensive TCAS II also provides pilots with resolution advisories (RAs) when needed. The system determines the course of each aircraft and whether it is climbing, descending or flying straight and level. TCAS II then issues an RA advising the pilots to execute the type of evasive maneuver necessary to avoid the other aircraft, such as “Climb” or “Descend.” If both planes are equipped with TCAS II, then the two computers offer deconflicting RAs. In other words, the pilots do not receive advisories to make maneuvers that would effectively cancel each other out, resulting in a continued threat.
Thus, the collision avoidance logic for TCAS II takes the collected data on the flight patterns of other aircraft and determines whether any of them present a potential collision threat. The overall system does not limit itself to the display of other planes, like a conventional radar screen, but offers warnings and solutions in the form of traffic advisories (TAs) and resolution advisories (RAs).
Traffic alert and Collision Avoidance Systems (TCAS) functionality can be improved with the GPS positioning capabilities of the ADS-B system. Such GPS position information will aid TCAS in determining more precise range and bearing at longer ranges. With greater precision, commercial aircraft can achieve higher safety levels and perform enhanced operational flying concepts such as in-trail climbs/descents, reduced vertical separation, and closely sequenced landings.
Additionally, GPS-based air traffic control systems, such as ADS-B, can also be used to extend traffic surveillance over greater distances. Previous technology limited surveillance ranges to a maximum of about 40 nautical miles (nm). ADS-B, since it does not require an active TCAS interrogation to determine range and bearing, will not be subject to a power limitation. As a result, in general, the ADS-B receiver capability determines surveillance range. For example, if the ADS-B receiver can process an ADS-B transmission out to 100 nm, then 100 nm is the effective range.
Currently, ADS-B technology is limited in deployment to a select, but constantly growing, number of aircraft while TCAS is a more industry wide standard. As such, the vast majority of current aircraft will rely solely on TCAS intruder data as a means of identifying and avoiding other aircraft within a specified range. Aircraft that are currently equipped with both ADS-B and TCAS technology can benefit from both data streams if the information can be correlated and integrated to provide for more accurate and precise identification of intruders/targets.
Due to variances in accuracy and resolution, the ADS-B and TCAS data for a single intruder aircraft will not be identical. Thus, uncorrelated ADS-B and TCAS data will typically result in two distinct symbols displayed on the CDTI. Therefore, a need exists to provide an on-board correlation method that will allow for a common intruder aircraft to be displayed as a single symbol on a CDTI. Correlation of the data serves to reduce display clutter or confusion that might arise from the display of multiple position symbols for the same intruder/target aircraft.